PART 1: Introduction
In order to assess current expert judgments about the radiative forcings produced by
aerosols, we are asking a number of leading experts to provide us with a series of
quantitative judgments. It is our hope to publish the results in time to be used in the
preparation of Chapter 2 of WG1 of the 4th IPCC Assessment.
We very much appreciate your willingness to assist us in this undertaking.
In reporting the results of this study, we will list the experts who participated, but we
will not identify individual experts with specific responses.
If you would like to see examples of previous work of this sort that we have done,
you could look at:
M. Granger Morgan, Samuel C. Morris, Max Henrion and Deborah A. L. Amaral, "Uncertainty
in Environmental Risk Assessment: A case study involving sulfur transport and health
effects", Environmental Science and Technology, 19, 662-667, 1985 August.
M. Granger Morgan and David Keith, "Subjective Judgments by Climate Experts,"
Environmental Science and Technology, 29(10), 468A-476A, October 1995.
M. Granger Morgan, Louis F. Pitelka and Elena Shevliakova, "Elicitation of Expert Judgments
of Climate Change Impacts on Forest Ecosystems," Climatic Change, 49, 279-307, 2001.
If you have questions about this project, please don't hesitate to contact us:
Prof. M. Granger Morgan
Carnegie Mellon University
e-mail:
gm5d @andrew.cmu.edu
US telephone:
412-268-2672
Prof. David Keith
University of Calgary
e-mail:
[email protected]
CA telephone:
403-220-6154
www.epp.cmu.edu
www.ucalgary.ca/~keith/
-32 -
Prof. Peter Adams
Carnegie Mellon
University
e-mail:
[email protected]
US telephone:
412-268-5624
www.epp.cmu.edu
PART 2: Forcings We Will Ask About
In this survey, we will ask you to make judgments about the following four different
aerosol-related forcings. In all cases, we are concerned with the anthropogenic
aerosol forcing, that is the difference between present day and pre-industrial
radiative flux at the top of the atmosphere due to aerosols.
aerosols 
indirect
effects
direct
effects
direct aerosol effect
(scattering and
absorption
from aerosols)
semi-direct aerosol
effect
(change in cloud
distribution due to local
heating from black
carbon absorption)
first aerosol indirect
effect
(cloud brightness
effect)
second aerosol indirect
effect
(cloud lifetime effect)
We define the four terms as follows:
Direct aerosol effect: change in radiative flux by scattering and absorption of
unactivated aerosol particles in the absence of any other climate changes or
feedbacks.
Semi-direct aerosol effect: change in radiative flux resulting from a change in
cloud distribution because of local heating by absorptive (e.g. black carbon)
aerosols.
First aerosol indirect effect (brightness): change in cloud reflectivity resulting
from a change in concentration of cloud condensation nuclei holding other
cloud properties constant (e.g. total liquid water and cloud cover).
Second aerosol indirect effect (lifetime): change in cloud cover/lifetime
resulting from a change in cloud condensation nuclei.
We will work our way through these four forcings one at a time. Then, since there
may be correlations among your answers, we'll also give you a chance to estimate
the total forcing from aerosols.
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We hope you will try hard to answer all the questions – if you are less sure about
some answers, then obviously feel free to indicate large uncertainty. If, however,
you absolutely feel you can't answer a question, skip to the next part. If you choose
to skip the first question (Part 4: relating to the direct aerosol effect), please read it
carefully nonetheless, as it outlines a specific process for reducing bias and
overconfidence in your answers that you should follow in subsequent answers.
Before we start asking questions, we want to show you a few examples from the
literature in order to caution you about the risks of overconfidence in judgments of
this type.
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PART 3: The Problems of Bias and Overconfidence
In asking you for your judgments, we have to be concerned about very strong
evidence in the literature that shows that people, including experts, often display
considerable overconfidence when asked to make subjective probabilisitic
judgments. That is, they produce probability distributions that are too narrow.
The figure below illustrates this problem. In 21 separate studies, well educated
people were asked to make judgments about the value of a large number of known
quantities (such as the length of the Panama Canal). They were also asked to
provide a 98% confidence interval on those judgments. The proportion of the time
that the true answers lay outside the 98% confidence interval that the respondents
had given, which of course should have been 2%, in fact looked like this (each box
in the histogram reports the results of a separate study, several of which had more
than 1000 participants):
0%
10%
20%
30%
40%
50%
60%
Percentage of estimates in which the true value
lay outside of the respondent’s assessed
98% confidence interval.
For details see Morgan and Henrion, Uncertatinty , Cambridge Univ. Press, 1990, pg 117.
Laypeople are not the only ones subject to overconfidence. Consider, for example,
the history of estimates of the speed of light:
Because of the problem of over confidence, and because of some other issues such
as the cognitive heuristic known as "anchoring and adjustment," when we first ask
you to provide probability estimates we will go about it in a somewhat indirect way,
asking first for upper and lower bounds before asking questions about more central
values. If you are interested, you can find a more detailed discussion of these
issues, and the subject of designing expert elicitations, in Chapters 6 and 7 of
Morgan and Henrion, Uncertainty (Cambridge, 1990).
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PART 4: Questions About the Direct Aerosol Effect
Direct aerosol effect: change
in radiative flux by scattering
and absorption of unactivated
aerosol particles in the absence
of any other climate changes or
feedbacks.
Before we ask you to make judgments about
the current average global magnitude of
radiative forcing due to the direct aerosol
effect caused by anthropogenic activities, we
would like you to consider the factors which
contribute to your uncertainty about the value
of this quantity.
We realize you can probably just list those factors, but to assure completeness
please review the list of factors below which might contribute uncertainty to your
judgment. Suppose we could do research that would eliminate or drastically reduce
uncertainty associated with each of these factors. (We realize some of these factors
are irrelevant for some of the forcings we'll ask you about but for simplicity we'll use
the same list throughout.)
Which three of these factors would most reduce your uncertainty about the value of
the direct aerosol effect if you could fully understand their influence? Write a 1 in
front of the factor which would do the most, if clarified, to reduce your uncertainty; a
2 in front of the factor which would do the next most; and a 3 in front of the factor
which would do the next most to reduce your overall uncertainty. If you want you
may rank more than three factors, and may give more than one factor a given rank.
Remember, the question is not which factor most influences the value of the direct
aerosol effect, but rather which factor most influences the value's uncertainty.
Emissions
Anthropogenic mass emission rate (<10microns) of all
aerosol species and precursors
Aerosol Scattering / Absorption-Related Properties
Deliquescence / crystallization state of aerosols
Natural mass emission rate (<10microns) of all
aerosol species and precursors
Aerosol water uptake
Aerosol single-scattering albedo
Size distribution of primary particles
Black carbon mixing state (externally vs. internally
mixed)
Composition and properties of primary emissions
(e.g. hygroscopicity or CCN activity of carbonaceous
emissions)
Others:
Others:
Aerosol-Cloud Interactions
CCN activity of carbonaceous particles
Atmospheric Processing: Factors that Relate
Emissions to Ambient Aerosol Burdens and
Properties
Deposition efficiency of aerosol particles
Surfactant properties of carbonaceous aerosols
In-cloud supersaturations
Production rate of condensable gases from
Black carbon heating rates local to clouds
precursors (i.e. SO 2 oxidation to SO 4 =
and secondary organic aerosol formation)
Sensitivity of cloud droplet number concentrations to
CCN
Coagulation rates
Sensitivity of precipitation rates to CCN
New particle formation (i.e. aerosol nucleation)
Role of “giant CCN”
Heterogeneous oxidation of carbonaceous
particles
Ice nucleation properties of aerosols
Sensitivity of ice clouds to ice nuclei concentrations
Aerosol mixing processes (conversion of
externally mixed particles to internally mixed)
General knowledge of clouds: dynamics, microphysics,
amount, distribution
Others:
Others:
-32 -
Now we are ready to ask you a set of questions to learn your views about the
uncertainty associated with the global average radiative forcing by the direct
aerosol effect measured in W/m².
In subsequent cases, we'll just ask you to construct a box plot like this:
However, for this first case we ask for your patience as we walk you through stepby-step.
Let's start with the absolute upper bound. Could the global average radiative
forcing from the direct aerosol effect be positive?
 No
 Yes

If you answered yes, how large could the positive forcing be? We are looking for an
absolute bound on your estimate:
+
W/m²
-32 -
If you answered no, what is the smallest negative value (including zero) that the
negative forcing could be? Again, we are looking for an absolute bound on your
estimate:
W/m²
Suppose that a decade from now the value is known with precision and it turns out
to fall several percent (of the box plot length) higher up, i.e. in the direction of more
positive forcing. Can you provide an explanation of how that might be possible? If
so, please briefly elaborate:
If you gave an explanation, please reconsider your estimate of the absolute upper
bound – from now on we will drop the sign and let you write it in.
W/m²
Now that you have set an absolute upper bound, we'd like an upper 5% confidence
limit. Please estimate a level of forcing for which there is a 95% chance that the
actual value of the magnitude of the global average direct aerosol effect lies below
(in the box plot diagram) the value you give:
W/m²
Please remember to indicate the sign.
-32 -
Now please estimate the absolute lower bound on the magnitude of the direct
aerosol effect (i.e., the most negative value that the forcing might take on):
W/m²
Suppose that a decade from now the value is known with precision and it turns out
to fall several percent (of the box plot length) lower down than the number you just
gave. Can you provide an explanation of how that might be possible? If so, please
briefly elaborate:
If you gave an explanation, please reconsider your estimate of the absolute lower
bound:
W/m²
Again, now that you have set an absolute lower bound, we'd like a 95% confidence
limit. Please estimate a level of forcing for which there is a 95% chance that the
actual value of the magnitude of the global average direct aerosol effect lies above
(in the box plot diagram) the value you give:
W/m²
Remember that people tend to be overconfident, so don't make your distribution too
narrow. Feel free to go back and spread your bounds if on reflection you think they
might be too tight.
Now we'd like the value for your lower 75% confidence interval. Please estimate a
level of forcing for which there is a 75% chance that the actual value of the
magnitude of the global average direct aerosol effect lies above (in the box plot
diagram) the value you give.
W/m²
-32 -
-32 -
Finally, we want your value for the upper 25% confidence interval. Please
estimate a level of forcing for which there is a 75% chance that the actual value of
the magnitude of the average direct aerosol effect lies below (in the box plot
diagram) the value you give:
W/m²
Lastly, what is your best estimate of the global average forcing associated with the
direct aerosol effect?
W/m²
When a distribution is asymmetric there is a difference between the mean and the
median. Can you tell us which you gave as your "best estimate"?
 The mean.
 The median.
 Doesn't matter, I think they are about the same.
 Darned if I know, that just feels about right.
-32 -
-32 -
To avoid having to go through all this laborious process for the other three
parameters, we'd like to provide you with a simpler response mode.
In the left side of the diagram below, we have again reproduced our example box
plot. Please go back to your answers on the previous pages and transcribe them
into a box plot constructed on the scale on the right to show your estimate of the
radiative forcing associated with the current global average value of the direct
aerosol effect. Note that to accommodate a wide range of answers to this and
subsequent questions we have made the scale somewhat non-linear
-32 -
Suppose we were to come back to you in 20 years and ask this question again.
Consider the full range of your uncertainty from lower to upper bound. What is the
probability that after 20 years of additional research at current levels of support the
outer tails of your box plot for the global average value of the direct aerosol
effect…..
Please enter a separate probability for each of the four contingencies.
______ probability that it will have gotten longer (i.e. taller)
______ probability that it will have gotten shorter by 0 to 50%
______ probability that it will have gotten shorter by 50% to 80%
______ probability that it will have gotten shorter by more than 80%
total probability = 1.0
-32 -
PART 5: Questions About the Semi-Direct Aerosol Effect
Semi-direct aerosol effect:
change in radiative flux
resulting from a change in
cloud distribution because of
local heating by black carbon
aerosols.
Before we ask you to make judgments about
the current global average magnitude of the
semi-direct aerosol effect caused by
anthropogenic activities, we would again like
you to consider the factors which contribute
to your uncertainty about the value of this
quantity.
Below is the same list of factors which might contribute uncertainty to your judgment
of the value of the semi-direct aerosol effect. Suppose we could do research to
eliminate or drastically reduce uncertainty associated with each of these factors.
Which three of these factors would most reduce your uncertainty about the value of
the semi-direct aerosol effect if you could fully understand their influence? Write a 1
in front of the factor which would do the most, if clarified, to reduce your uncertainty;
a 2 in front of the factor which would do the next most; and a 3 in front of the factor
which would do the next most to reduce your overall uncertainty. If you want, you
may rank more than three factors, and may give more than one factor a given rank.
Remember, the question is not which factor most influences the value of the direct
aerosol effect, but rather which factor most influences the value's uncertainty.
Emissions
Anthropogenic mass emission rate (<10microns) of all
aerosol species and precursors
Aerosol Scattering / Absorption-Related Properties
Deliquescence / crystallization state of aerosols
Natural mass emission rate (<10microns) of all
aerosol species and precursors
Aerosol water uptake
Aerosol single-scattering albedo
Size distribution of primary particles
Black carbon mixing state (externally vs. internally
mixed)
Composition and properties of primary emissions
(e.g. hygroscopicity or CCN activity of carbonaceous
emissions)
Others:
Others:
Aerosol-Cloud Interactions
CCN activity of carbonaceous particles
Atmospheric Processing: Factors that Relate
Emissions to Ambient Aerosol Burdens and
Properties
Deposition efficiency of aerosol particles
Surfactant properties of carbonaceous aerosols
In-cloud supersaturations
Production rate of condensable gases from
Black carbon heating rates local to clouds
precursors (i.e. SO 2 oxidation to SO 4 =
and secondary organic aerosol formation)
Sensitivity of cloud droplet number concentrations to
CCN
Coagulation rates
Sensitivity of precipitation rates to CCN
New particle formation (i.e. aerosol nucleation)
Role of “giant CCN”
Heterogeneous oxidation of carbonaceous
particles
Ice nucleation properties of aerosols
Sensitivity of ice clouds to ice nuclei concentrations
Aerosol mixing processes (conversion of
externally mixed particles to internally mixed)
General knowledge of clouds: dynamics, microphysics,
amount, distribution
Others:
Others:
-32 -
-32 -
Now please use the scale below to construct a box plot to describe your uncertainty
about the value of the current global average magnitude of the semi-direct
aerosol effect.
To minimize the risk of overconfidence, please start by drawing short horizontal lines
to denote the lower and upper extreme values. Ask yourself if you could explain
smaller and larger values if they were found in the future, and if so, revise your
bounds accordingly. Then fill in the other elements of the box plot (X=5%;
=25%; =best estimate;
=75%; X = 95%).
+3
+2
+1
Radiative
0 forcing
in W/m2
-1
-2
-3
-4
-5
-6
-7
-32 -
Can you tell us which you gave as your "best estimate"?
 The mean.
 The median.
 Doesn't matter, I think they are about the same.
 Darned if I know, that just feels about right.
Suppose we were to come back to you in 20 years and ask this question again.
Consider the full range of your uncertainty from lower to upper bound. What is the
probability that after 20 years of additional research at current levels of support the
outer tails of your box plot for the semi-direct aerosol effect…..
Please enter a separate probability for each of the four contingencies.
______ probability that it will have gotten longer (i.e. taller)
______ probability that it will have gotten shorter by 0 to 50%
______ probability that it will have gotten shorter by 50% to 80%
______ probability that it will have gotten shorter by more than 80%
total probability = 1.0
-32 -
PART 6: Questions About First Aerosol Indirect Effect
First aerosol indirect effect
(brightness): change in cloud
reflectivity resulting from a
change in cloud condensation
nuclei holding other cloud
properties constant (e.g. total
liquid water and cloud cover).
Before we ask you to make judgments
about the current global average
magnitude of the first aerosol indirect
effect caused by anthropogenic activities,
we would again like you to consider the
factors which contribute to your
uncertainty about the value of this
quantity.
Below is the same list of factors which might contribute uncertainty to your judgment
of the value of the first indirect effect.
Suppose we could do research to eliminate or drastically reduce uncertainty
associated with each of these factors. Which three of these factors would most
reduce your uncertainty about the value of the first indirect effect if you could fully
understand their influence? Write a 1 in front of the factor which would do the most,
if clarified, to reduce your uncertainty; a 2 in front of the factor which would do the
next most; and a 3 in front of the factor which would do the next most to reduce your
overall uncertainty. Remember, the question is not which factor most influences the
value, but rather which factor most influences the value's uncertainty.
Emissions
Anthropogenic mass emission rate (<10microns) of all
aerosol species and precursors
Aerosol Scattering / Absorption-Related Properties
Deliquescence / crystallization state of aerosols
Natural mass emission rate (<10microns) of all
aerosol species and precursors
Aerosol water uptake
Aerosol single-scattering albedo
Size distribution of primary particles
Black carbon mixing state (externally vs. internally
mixed)
Composition and properties of primary emissions
(e.g. hygroscopicity or CCN activity of carbonaceous
emissions)
Others:
Others:
Aerosol-Cloud Interactions
CCN activity of carbonaceous particles
Atmospheric Processing: Factors that Relate
Emissions to Ambient Aerosol Burdens and
Properties
Deposition efficiency of aerosol particles
Surfactant properties of carbonaceous aerosols
In-cloud supersaturations
Production rate of condensable gases from
Black carbon heating rates local to clouds
precursors (i.e. SO 2 oxidation to SO 4 =
and secondary organic aerosol formation)
Sensitivity of cloud droplet number concentrations to
CCN
Coagulation rates
Sensitivity of precipitation rates to CCN
New particle formation (i.e. aerosol nucleation)
Role of “giant CCN”
Heterogeneous oxidation of carbonaceous
particles
Ice nucleation properties of aerosols
Sensitivity of ice clouds to ice nuclei concentrations
Aerosol mixing processes (conversion of
externally mixed particles to internally mixed)
General knowledge of clouds: dynamics, microphysics,
amount, distribution
Others:
Others:
-32 -
-32 -
Now please use the scale below to construct a box plot to describe your uncertainty
about the value of the current global average magnitude of the first aerosol
indirect effect.
To minimize the risk of overconfidence, please start by drawing short vertical lines to
denote the lower and upper extreme values. Ask yourself if you could explain
smaller and larger values if they were found in the future, and if so, revise your
bounds accordingly. Then fill in the other elements of the box plot (X=5%;
=25%; =best estimate;
=75%; X = 95%).
+3
+2
+1
Radiative
0 forcing
in W/m2
-1
-2
-3
-4
-5
-6
-7
-32 -
Can you tell us which you gave as your "best estimate"?
 The mean.
 The median.
 Doesn't matter, I think they are about the same.
 Darned if I know, that just feels about right.
Suppose we were to come back to you in 20 years and ask this question again.
Consider the full range of your uncertainty from lower to upper bound. What is the
probability that after 20 years of additional research at current levels of support the
outer tails of your box plot for first aerosol indirect effect…..
Please enter a separate probability for each of the four contingencies.
______ probability that it will have gotten longer (i.e. taller)
______ probability that it will have gotten shorter by 0 to 50%
______ probability that it will have gotten shorter by 50% to 80%
______ probability that it will have gotten shorter by more than 80%
total probability = 1.0
-32 -
PART 7: Questions About the Second Aerosol Indirect Effect
Second aerosol indirect
effect (lifetime): change in
cloud cover/lifetime resulting
from a change in cloud
condensation nuclei.
Before we ask you to make judgments
about the current global average magnitude
of the second aerosol indirect effect caused
by anthropogenic activities, we would again
like you to consider the factors which
contribute to your uncertainty about the
value of this quantity.
Below is the same list of factors which might contribute uncertainty to your judgment
of the value of the second indirect effect.
Suppose we could do research to eliminate or drastically reduce uncertainty
associated with each of these factors. Which three of these factors would most
reduce your uncertainty about the value of the second indirect effect if you could fully
understand their influence? Write a 1 in front of the factor which would do the most,
if clarified, to reduce your uncertainty; a 2 in front of the factor which would do the
next most; and a 3 in front of the factor which would do the next most to reduce your
overall uncertainty. Remember, the question is not which factor most influences the
value, but rather which factor most influences the value's uncertainty.
Emissions
Anthropogenic mass emission rate (<10microns) of all
aerosol species and precursors
Aerosol Scattering / Absorption-Related Properties
Deliquescence / crystallization state of aerosols
Natural mass emission rate (<10microns) of all
aerosol species and precursors
Aerosol water uptake
Aerosol single-scattering albedo
Size distribution of primary particles
Black carbon mixing state (externally vs. internally
mixed)
Composition and properties of primary emissions
(e.g. hygroscopicity or CCN activity of carbonaceous
emissions)
Others:
Others:
Aerosol-Cloud Interactions
CCN activity of carbonaceous particles
Atmospheric Processing: Factors that Relate
Emissions to Ambient Aerosol Burdens and
Properties
Deposition efficiency of aerosol particles
Surfactant properties of carbonaceous aerosols
In-cloud supersaturations
Production rate of condensable gases from
Black carbon heating rates local to clouds
precursors (i.e. SO 2 oxidation to SO 4 =
and secondary organic aerosol formation)
Sensitivity of cloud droplet number concentrations to
CCN
Coagulation rates
Sensitivity of precipitation rates to CCN
New particle formation (i.e. aerosol nucleation)
Role of “giant CCN”
Heterogeneous oxidation of carbonaceous
particles
Ice nucleation properties of aerosols
Sensitivity of ice clouds to ice nuclei concentrations
Aerosol mixing processes (conversion of
externally mixed particles to internally mixed)
General knowledge of clouds: dynamics, microphysics,
amount, distribution
Others:
Others:
-32 -
-32 -
Now please use the scale below to construct a box plot to describe your uncertainty
about the value of the current global average magnitude of the second aerosol
indirect effect.
To minimize the risk of overconfidence, please start by drawing short vertical lines to
denote the lower and upper extreme values. Ask yourself if you could explain
smaller and larger values if they were found in the future, and if so, revise your
bounds accordingly. Then fill in the other elements of the box plot (X=5%;
=25%; =best estimate;
=75%; X = 95%).
+3
+2
+1
Radiative
0 forcing
in W/m2
-1
-2
-3
-4
-5
-6
-7
-32 -
Can you tell us which you gave as your "best estimate"?
 The mean.
 The median.
 Doesn't matter, I think they are about the same.
 Darned if I know, that just feels about right.
Suppose we were to come back to you in 20 years and ask this question again.
Consider the full range of your uncertainty from lower to upper bound. What is the
probability that after 20 years of additional research at current levels of support the
outer tails of your box plot for the second aerosol indirect effect…..
Please enter a separate probability for each of the four contingencies.
______ probability that it will have gotten longer (i.e. taller)
______ probability that it will have gotten shorter by 0 to 50%
______ probability that it will have gotten shorter by 50% to 80%
______ probability that it will have gotten shorter by more than 80%
total probability = 1.0
-32 -
PART 8: Questions About the Total Aerosol Forcing
If you have answered all four of the sets of preceding questions, we might be able to
compute the total forcing due to aerosols caused by anthropogenic activities as the
sum of the four distributions you provided. However, if there were correlation among
your different answers, that sum would probably overestimate the total uncertainty.
Please check one of the following options:
 The sum of my four distributions will not accurately reflect my view of the total
aerosol forcing because there is likely correlation among the distributions (in
which case please complete this section).
 I did not feel that I could answer all four of the preceding sets of questions (in
which case please complete this section).
 You can just add my four distributions together to get my estimate of total
aerosol forcing – that is, the four distributions are not correlated (in which case,
skip this section and jump to Part 9 on page 29).
Now please use the scale on the following page to construct a box plot to describe
your uncertainty about the value of the total aerosol forcing.
-32 -
-32 -
Total Aerosol Forcing
To minimize the risk of overconfidence, please start by drawing short horizontal lines
to denote the lower and upper extreme values. Ask yourself if you could explain
smaller and larger values if they were found in the future, and if so, revise your
bounds accordingly. Then fill in the other elements of the box plot (X=5%;
=25%; =best estimate;
=75%; X = 95%).
+3
+2
+1
Radiative
0 forcing
in W/m2
-1
-2
-3
-4
-5
-6
-7
-32 -
Can you tell us which you gave as your "best estimate"?
 The mean.
 The median.
 Doesn't matter, I think they are about the same.
 Darned if I know, that just feels about right.
Suppose we were to come back to you in 20 years and ask this question again.
Consider the full range of your uncertainty from lower to upper bound. What is the
probability that after 20 years of additional research at current levels of support the
outer tails of your box plot for the current global average magnitude of the total
aerosol forcing (sum of all direct and indirect forcings):
Please enter a separate probability for each of the four contingencies.
______ probability that it will have gotten longer (i.e. taller)
______ probability that it will have gotten shorter by 0 to 50%
______ probability that it will have gotten shorter by 50% to 80%
______ probability that it will have gotten shorter by more than 80%
total probability = 1.0
-32 -
PART 9: Question About Surface Forcing
It has been pointed out (for example by Ramanathan et al., 2001) that absorbing
aerosols cause large surface forcings that affect the hydrological cycle but are not
captured by total aerosol forcing at the top of the atmosphere. Thus, before we end,
we would like you to use the scale below to estimate the uncertainty in the surface
forcing by aerosols. Note that the magnitude of the atmospheric forcing is implied by
the top of atmosphere forcing and surface forcing. Please follow the same procedure
you've used in constructing the previous plots.
+3
+2
+1
Radiative
0 forcing
in W/m2
-1
-2
-3
-4
-5
-6
-7
V. Ramanathan, et al., “Atmosphere - Aerosols, climate, and the hydrological cycle,” Science, 294 (5549);
2119-2124, 2001.
-32 -
PART 10: Your Expertise
Please provide us with your self evaluation of your expertise in answering the
questions we have posed in the previous sections.
Direct aerosol effect:
not
familiar
with this
literature

general
knowledge of
the relevant
literatures
among the handful
of top experts
in the world
------------------------------------
Semi-direct aerosol effect:
not
familiar
with this
literature

general
knowledge of
the relevant
literatures
among the handful
of top experts
in the world
------------------------------------
First aerosol indirect effect:
not
familiar
with this
literature

general
knowledge of
the relevant
literatures
among the handful
of top experts
in the world
------------------------------------
Second aerosol indirect effect:
not
familiar
with this
literature

general
knowledge of
the relevant
literatures
among the handful
of top experts
in the world
------------------------------------
Total aerosol forcing (sum of all direct and indirect forcings):
not
familiar
with this
literature

general
knowledge of
the relevant
literatures
among the handful
of top experts
in the world
------------------------------------
Surface aerosol forcing:
not
familiar
with this
literature

general
knowledge of
the relevant
literatures
among the handful
of top experts
in the world
------------------------------------
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On which types of experience and knowledge did you base your answers? Please
rank the following options from 1 (most important in shaping your judgments) to 6
(least important in shaping your judgments).
laboratory studies,
local measurement studies,
global measurement studies (e.g. satellite retrievals),
local modeling studies (e.g. mesoscale or cloud resolving model studies),
global modeling studies (e.g. GCM experiments),
general assessment of the literature.
Please tell us if your uncertainty ranges were mostly constrained by “forward” or
“reverse” calculations. In “forward” calculations, observed or modeled aerosols are
used to calculate radiative forcings. In “reverse” calculations, the aerosol forcing is
the “missing” forcing required for a GCM simulation of the past to reproduce the
observed temperature record.
Forward calculations
Reverse calculations
If you are using “forward” calculations, are your uncertainty ranges mostly
constrained by observations or models? In the modeling approach, present and preindustrial aerosol levels and properties result from a model simulation using
corresponding emissions (realizing models ought to be informed by and tested
against observations). In the observational approach, aerosol levels and properties,
and/or forcings are deduced from in-situ and remote sensing observations.
Models
Observations
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Thanks very much for your help in this project.
Please return this booklet
in the envelope provided to:
Prof. M. Granger Morgan
Department of Engineering
and Public Policy
129 Baker Hall
Carnegie Mellon University
Pittsburgh, PA, 15213 USA
Work supported by NSF under cooperative agreement SES-034578.
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